1. Field of the Invention
The present invention relates generally to a multi-layered golf ball having an inner core, at least one intermediate layer, and outer cover; wherein the outer cover is made from an ultra-low melt index (ULMI) thermoplastic material. The outer cover is made by an in-molding coating process that involves applying a thin layer of ULMI thermoplastic material to the interior surface of the cover mold members.
2. Brief Review of the Related Art
Today, golf balls having multi-layered designs are commonly used by professional and recreational golfers. For example, three-piece balls having an inner core, at least one intermediate layer surrounding the core, and an outer cover are popular. Different materials are used to make each of these layers. The materials are designed to impart more desirable playing performance properties to the golf ball.
For instance, a variety of materials may be used to make the inner core of the ball, particularly natural and synthetic rubbers such as styrene butadiene, polybutadiene, poly(cis-isoprene), and poly(trans-isoprene). The core is the primary source of resiliency for the golf ball and is often referred to as the engine of the ball. The ball may include one or more intermediate layers made from thermoplastic or thermoset resins such as polyamides, polyesters, ethylene-based ionomers, polyurethanes, and polyureas. As used herein, the term, “intermediate layer” means a layer of the ball disposed between the core and cover. The intermediate layer may be considered an outer core layer or inner cover layer or any other layer disposed between the inner core and outer cover of the ball. The intermediate layer also may be referred to as a casing or mantle layer. The intermediate layers are designed to impart special properties to the ball. For example, the intermediate layers often are made of materials that prevent moisture from penetrating into the core. An inner cover may be made of olefin-based ionomer copolymers that impart hardness to the ball. These polymers contain inter-chain ionic bonding and generally refer to ionic copolymers of an olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic or acrylic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available ionomer resins are known in the industry and include numerous resins sold under the trademarks, Surlyn® (DuPont) and Escor® and Iotek® (Exxon). These ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, type of metal ion, amount of acid, degree of neutralization, additives, and other properties.
The outer cover of the ball is designed to protect the core and provides the ball with durability, toughness, and cut/tear-resistance. The cover layer may be single or multi-layered. Conventional cover materials include polyurethanes, polyureas, and blends thereof, as well as olefin-based ionomer copolymers. The combination of core, intermediate layer(s), and cover provides the golf ball with its targeted performance properties.
For example, the resiliency or coefficient of restitution (“COR”) of a golf ball (or golf ball subassembly such as a core) means the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of air cannon into a rigid plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize the COR under these conditions. Balls (or cores) with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance. In general, the COR of the ball will increase as the hardness of the ball is increased. The test methods for measuring the COR are described in further detail below.
It is well known that hard golf balls having relatively thick, hard outer covers can be made, and such balls generally have good durability, toughness, and impact-resistance. For example, hard ionomer resins can be used to make such covers. These thick-covered, ionomeric golf balls generally are harder and more resistant to wear and tear. The thick outer cover protects the core and such balls have good impact durability and cut/tear-resistance. However, these golf balls also can be overly stiff, and they tend to have low spin. Players tend to experience a harder feel when their club makes contact with such stiff balls. The player senses less control. The player has generally a less natural and comfortable sensation when striking such thick-covered, hard golf balls versus thin-covered, soft balls.
Thus, the golf industry has looked to develop golf balls having relatively thin cover layers. For example, golf balls having covers made from relatively soft polyurethanes, polyureas, and polyurethane/urea blends have been developed in recent years. For example, Hebert et al., U.S. Pat. Nos. 6,132,324 and 5,885,172 disclose a method of forming a multi-layered golf ball comprising a core, inner cover layer, and outer cover layer. A castable reactive liquid polyurethane or polyurea material is introduced into mold cavities and then a ball subassembly (core and inner cover layer) is placed in one mold cavity. The upper and lower mold cavities are joined together. The polyurethane or polyurea material in the cavities encapsulates the ball subassembly and forms a thin cover for the ball.
In Lutz et al., U.S. Pat. Nos. 6,783,808 and 6,706,332 a method of coating a thin-layered over a golf ball component is provided. The method involves providing a polymer material; creating a polymer particulate from the polymer material; fluidizing the polymer particulate; and coating the golf ball component with a thin layer of the polymer material by placing the golf ball component within the fluidized particulate. Suitable polymers are described as including vinyl resins; polyolefins; polyurethanes; polyureas; polyamides; acrylic resins; and other thermoplastics and thermosets.
Conventional thin covers provide the ball with a softer feel, and the player can place a spin on the ball and better control its flight pattern. The softer cover feels more natural. Players sense more control with such softer, relatively thin-covered golf balls. There are drawbacks, however, with such thin-covered golf balls, because the balls tend to have less durability, toughness, and cut/tear-resistance. The ball may appear excessively worn with scuff marks, cuts, and tears after continuous play on the golf course. In addition, there can be drawbacks with using conventional methods such as casting and reaction injection molding (“RIM”) to form thin cover layers. For example, casting processes may produce undesirable waste, and RIM mold parts may be difficult to position to achieve a uniform layer and leave pin marks on the cores or golf ball subassemblies. Thin layers may also be sprayed on the golf ball assemblies; however, spray applicators or nozzles can be clogged and the liquid compositions to be sprayed may also have undesirably high volatile organic components (VOC).
It would be desirable to make thin covers for golf balls using ULMI polymers, because these polymers could provide the ball with a combination of optimum properties. For example, these polymers could provide the ball with high impact durability, toughness, and cut/tear-resistance. Such covers, in combination with the rubber cores, would impart high resiliency to the golf balls. This would allow players to generate greater initial ball velocity off the tee and achieve greater distance. At the same time, the relatively thin cover layers would provide the ball with a comfortable softness and natural feeling. However, manufacturing golf ball covers from ULMI thermoplastic polymers is particularly difficult because of the relatively poor flow characteristics of these materials. Conventional molding techniques, as discussed above, have various drawbacks.
The present invention provides new methods for making golf balls having cover layers made from ULMI thermoplastic polymers. The resulting balls having desirable playing performance properties as well as other benefits and advantages.